This invention relates generally to a radio frequency transmitter. More specifically, the invention is related to a low power transmitter having particular utility for use in a remote keyless entry, alarm or similar system for a motor vehicle.
A remote system for a motor vehicle has a base unit in a vehicle and a remote unit carried by the operator. A remote unit is typically low power battery operated and quite small, designed about the size of keychain. Such remote units are generally configured to at least transmit a stream of binary data utilizing known radio frequency transmission modulation techniques. The most common technique utilized in this application is Amplitude Shift Keying (ASK). Experience has shown that ASK may exhibit performance limitations in certain radio frequency hostile environments resulting in reduced range and noise immunity.
One type of alternate radio frequency modulation technique which proves more robust than ASK is Frequency Shift Keying (FSK). Problematic of prior art FSK circuitry is frequency drift and limited separation of generated carrier frequencies. Commonly employed varactor diode frequency pulling techniques yield less than the full frequency separation range possible, exhibits temperature instability characteristic of solid state electronics and requires precise control of a reverse bias voltage applied thereto. Other techniques make use of a multiplicity of reactive element circuit paths for frequency pulling but likewise tend to be temperature drift sensitive and accomplish less than the full range of potential frequency separation. Multiple circuit paths and related switching circuitry reduce overall reliability and increase overall cost due to circuital complexity.